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Don't pick your 'favorites' for tuning. Don't pick your ideal 'demo' material for tuning.
For level setting, select a clean, busy, well-recorded, dynamic artist/song with activity across the entire bandwidth. It doesn't have to be as dense as Nickelback but that is on the right track. You also don't want a selection that falsely emphasizes or boosts one area. I prefer to stay away from music with synthesized bass. Neutral is better.
I don't listen to Dave Mathews but the first cut on Crash is good for tuning. It's recorded a bit hot and reveals clipping.

There's always the SMD dd-1 for gain setting. There is also a neat app from focal that might help u it's called "focal teach HD" really good app if ur not sure what to listen for when ur tuning ur system. Plus it's free too.

System Tuning

What is the definition of clipping, when it begins to distort? Are you tuning for max amount of volume without clipping? In other words should this be done in an open field/water far away from civilization?

Depends on who your neighbors are. The HS kids across the street from me like to jam rap music out of the garage around 3 pm before their parents get home. They are huge workout guys who lift weights and stuff while they jam. If it wasn't for the lyrics I wouldn't mind. Don't like the kids asking me "what's a mugga fugga nidda, Dad?"

May have to tune the boat stereo out in the driveway at 7 am Sunday morning and have a nice conversation with their parents.

What is the definition of clipping, when it begins to distort? Are you tuning for max amount of volume without clipping? In other words should this be done in an open field/water far away from civilization?

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An amplifier's job is to magnify the audio signal as a faithful replica of the incoming signal. Any amplifier has limits. Consider those limits as a fixed box with a definite ceiling and floor. When the signal is expanded beyond the boundaries of the fixed box, any portion of the signal exceeding the box is clipped off or flattened out. Now, the peaks are essentially missing. And that is distortion. The crest factor is reduced which means the music is compressed without the full dynamic contrast.
The signal changes form from one that is largely in transition to one that is more continuous. That creates a high risk of thermal induced speaker damage as the speaker is not given the opportunity to dissipate heat. An amplifier driven into clipping or compression begins to narrow in bandwidth. It can continue to put out more power but over a narrower range. An over-driven amplifier becomes increasingly inefficient consuming more power but with little additional power getting to the speakers.
So clipping not only sounds nasty but it has numerous byproducts. All of them are destructive.

With a test disc and a multimeter, you can find the clipped limits in total silence as long as you know the true power limits of an amplifier in advance. But if the specs are bogus, this doesn't work out. Or, the SMD or a handheld OScope can find clipping without audio.
This only establishes the maximum setting. To finish level setting and level matching, the system is going to have to play very loud somewhere.

Mmmmk...so then I am correct in my thought that clipping is distortion?

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Clipping is distortion, yes. It causes the amplifier's, (or other audio device) output waveform to change with respect to the input waveform. Let me show you graphically what David is saying, with the attached image.

At the very top of this picture you see a red squiggly line. This line represents the signal, (frequency-modulated alternating current) going into any audio device. To explain a little further, high sounds like come out of tweeters have squiggles that are very closely spaced, (left to right) and low sounds like come out of woofers are very widely spaced. There is a positive and a negative voltage component to the signal. We talk about the peak voltage of any individual "hump" as its AMPLITUDE. Speakers go in AND out... positive voltage makes the motor in the speaker push the cone out... negative voltage pulls the cone in. DOing this very quickly allows the speaker cone to vibrate the air quick enough that we perceive it as sound.

Now back to AMPLITUDE. We just said AMPLITUDE can be expressed in Volts. A higher power amplifier can provide higher absolute voltages at its speaker connections. It is called an amplifier as it will take the relatively small voltage signal from a CD player or radio and AMPLIFY it to a higher voltage. Any amplifier has a max positive and "max" negative voltage that it can put out. In the picture I drew, the theoretical amplifier has its peak AMPLITUDE at +12 and -12 volts. As David shared, these two extreme voltages can be used graphically to draw a box that the amplified waveform has to fit in. I have drawn a few boxes.

In the first box, (top down) I have literally a representation of an amplifier that is delivering an output signal that is exactly the same as the original top input signal. If we had an AC voltmeter we would be able to measure the voltage coming out of the radio and then measure the voltage coming out of the amp and see that they were the same. This represents an amp where the gain is set so low that it really does no amplification. If you were viewing the amplifier output on an oscilloscope it would look very similar to this. Make sense?

Next picture down we see the waveform is taller, and it fits more tightly between the top and bottom of the box. This represents again the output waveform of the amplifier. In this second picture the gain has been raised and the amplifier is really amplifying. If using that same AC voltmeter we measured the radio's output and then measured the amplifiers output, we would see that the amplifiers output voltage is greater; it is now acutally AMPLIFYING the input signal and making it louder. At the same time, the AMPLITUDE of the waveform is still less than the + and - 12 volts that the amplifier can make. Again, if you had an oscilloscope, you would see the same amplified squiggle. Nore the tops and bottoms of the waveform still form arcs; they are rounded with nice smooth curves... In geek terms, we can say that we still have a sinusoidal waveform. This is desirable and would indicate an amplifier whose gain was probably set darned close to right at least for the speakers it is driving, not considering final level matching which David points out.

Now look at the third picture. In the third picture we see an example of a waveform that has an AMPLITUDE greater than what the amplifier can deliver on its outputs. Again, we are hypothetically setting a max +/- voltage for this example at 12 volts, yet the wafeform if fully and sinusoidally were amplified, its peak AMPLITUDE would be about 16 volts. Quite simply the amplifier cannot do that... So, any part of the waveform that would be creater than +/-12V in amplitude is CLIPPED off. Literally CLIPPED, hence the terminology. The result is that if you looked at an oscilloscope at the output of a clipped amplifier, it would look like the bery bottom red squiggle, (box removed). See how the peaks are flattened? That is the CLIPPED signal....

Now David and I have slightly different ways to explain the damage caused, but the results are the same. Let me briefly try to touch on this:
When we look at the squiggle from a time-perspective, (from left to right) we can see that voltage is constantly changing. if you pick any point and draw a dot on any of the top three red squiggles, you can then see that to the left and to the right the voltage, (amplitude) is different. The voltage is ALWAYS going up, going down, or changing from up to down, (top of the curves). This changing voltage moves the speaker cone back and forth in a controlled manner, and turns the electrical energy into sound. CHANGING voltage makes sound.

Now go to the very bottom red squiggle and draw a dot right in the middle of one of the horizontal flat spots. What is the amplitude/voltage difference on either side of the dot you drew? It is the SAME, (Here is the part where David and I diverge as I am ONLY explaining in very layman's terms what is happening for a VERY small time window). For that very brief period of time where the output of the amplifier is flattened at max voltage, the amplifier puts out what on an oscilloscope looks like DC voltage. It also acts like DC voltage. DC voltage does not change, so it does not gently move a speaker cone in and out, it drives it right out to the very edge of its excursion, and then the speaker does not move. It is stuck out there. Consider taking a small speaker and connecting a 9-volt battery to it. VOltage is still moving theough the voice coil, yet the speaker is not moving, so the coil is sitting there just heating up.

I will defer back to David now, and acknowledge that a very heavily clipped amplifier is putting out an AC square wave, and is not really putting out DC, but the analogy is a good one to help explain that for those brief moments where an amplifiers output voltage is maxed out, it behaves and does damage the same way as if one were to sinply take a speaker and connect it to a car battery.....

This is very much a simplfie layman's terms explanation of what cliping is, but I hope that the simplfied explanation makes it clearer!